Jeremy J. Titman

Solid-state proton multiple-quantum NMR spectroscopy with fast magic angle spinning

Abstract The feasibility of multiple-quantum NMR spectroscopy with high resolution for protons in solids is explored. A new multiple-quantum excitation sequence suitable for use with fast magic angle spinning is described, and its performance is compared to that of both static and slow-spinning multiple-quantum methods. Modified sequences with scale the rate of development of the multiple-quantum coherences are also demonstrated, and two-dimensional double-quantum spectra of adamantane and polycarbonate are presented.

Pore with Gate: Enhancement of the Isosteric Heat of Adsorption of Dihydrogen via Postsynthetic Cation Exchange in Metal-Organic Frameworks

Three isostructural anionic frameworks {[(Hdma)(H(3)O)][In(2)(L(1))(2)]·4DMF·5H(2)O}(∞) (NOTT-206-solv), {[H(2)ppz][In(2)(L(2))(2)]·3.5DMF·5H(2)O}(∞) (NOTT-200-solv), and {[H(2)ppz][In(2)(L(3))(2)]·4DMF·5.5H(2)O}(∞) (NOTT-208-solv) (dma = dimethylamine; ppz = piperazine) each featuring organic countercations that selectively block the channels and act as pore gates have been prepared. The organic cations within the as-synthesized frameworks can be replaced by Li(+) ions to yield the corresponding Li(+)-containing frameworks {Li(1.2)(H(3)O)(0.8)[In(2)(L(1))(2)]·14H(2)O}(∞) (NOTT-207-solv), {Li(1.5)(H(3)O)(0.5)[In(2)(L(2))(2)]·11H(2)O}(∞) (NOTT-201-solv), and {Li(1.4)(H(3)O)(0.6)[In(2)(L(3))(2)]·4acetone·11H(2)O}(∞) (NOTT-209-solv) in which the pores are now unblocked. The desolvated framework materials NOTT-200a, NOTT-206a, and NOTT-208a display nonporous, hysteretic and reversible N(2) uptakes, respectively, while NOTT-206a and NOTT-200a provide a strong kinetic trap showing adsorption/desorption hysteresis with H(2). Single crystal X-ray analysis confirms that the Li(+) ions are either tetrahedrally (in NOTT-201-solv and NOTT-209-solv) or octahedrally (in NOTT-207-solv) coordinated by carboxylate oxygen atoms and/or water molecules. This is supported by (7)Li solid-state NMR spectroscopy. NOTT-209a, compared with NOTT-208a, shows a 31% enhancement in H(2) storage capacity coupled to a 38% increase in the isosteric heat of adsorption to 12 kJ/mol at zero coverage. Thus, by modulating the pore environment via postsynthetic cation exchange, the gas adsorption properties of the resultant MOF can be fine-tuned. This affords a methodology for the development of high capacity storage materials that may operate at more ambient temperatures.

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.

A convenient technique for the measurement and assignment of long-range carbon-13 proton coupling constants

Abstract A new technique is described which enables accurate values of heteronuclear long-range coupling constants to be obtained from highly distorted multiplets in two-dimensional correlation spectra. The technique uses a simple data fitting of the distorted multiplet with a model obtained from the conventional proton spectrum, and achieves high sensitivity by detecting the signals at the proton frequency.

Improved scalar shift correlation NMR spectroscopy in solids

New isotropic mixing sequences suitable for scalar correlation experiments in solids have been designed using symmetry principles similar to those employed in the construction of the C7 dipolar recoupling sequence. Compared with existing methods, the new isotropic mixing sequences are appropriate for use with faster MAS rates and show improved magnetization transfer efficiencies for carbon-13.

Two-dimensional MAS-NMR spectra which correlate fast and slow magic angle spinning sideband patterns

Abstract A new NMR experiment which allows a measurement of the chemical shift anisotropy (CSA) tensor under magic angle spinning (MAS) is described. This correlates a fast MAS spectrum in the ω 2 dimension with a sideband pattern in ω 1 in which the intensities mimic those for a sample spinning at a fraction of the rate ω r / N . This method is particularly useful for accurately measuring narrow shift anisotropies. Since the sidebands intensities in ω 1 are identical to those expected at ω r / N , standard methods can be used to extract the principal tensor components. The nature of the experiment is such that a minimal number of t 1 increments is required.

Structure and order in partially oriented solids by three‐dimensional magic angle spinning nuclear magnetic resonance spectroscopy

A new rotor‐synchronized carbon‐13 three‐dimensional magic angle spinning NMR experiment for the extraction of orientational distribution functions from partially ordered solid materials is presented. Problems with overlap of two‐dimensional spinning sideband patterns inherent in earlier methods are overcome by extension to three dimensions, allowing the degree of order for each resolvable carbon site to be measured. The new technique is demonstrated on a frozen smectic liquid‐crystalline side group polymer which shows an order gradient from the aligned mesogen to the disordered polymer main chain.

Mapping of motional heterogeneity in organic‐inorganic nanocomposite gels

Organic-inorganic nanocomposite gels comprising poly(ethyleneoxide) chains grafted between silica particles have been studied by electron paramagnetic and nuclear magnetic resonances. Motional heterogeneity has been investigated as a function of gelation and drying by paramagnetic probes dissolved in the organic phase or grafted onto the inorganic nodes. The organic phase exhibits marked variations in dynamic behavior with the liquid-like mobility of the polymer chains strongly hindered at the silica nodes. The conclusions of the EPR study are confirmed by liquid- and solid-state carbon-13 NMR measurements.

ASSESSMENT OF A METHOD FOR THE MEASUREMENT OF LONG-RANGE HETERONUCLEAR COUPLING-CONSTANTS

Abstract Recently ( J. Magn. Reson. 85 , 111–131, 1989) we described a method for the convenient measurement of long-range heteronuclear coupling constants using a combination of a proton-detected two-dimensional shift-correlation spectrum and a simple data-fitting procedure. This method enables accurate values of the long-range couplings to be extracted from the highly distorted cross peaks in the two-dimensional spectrum. In this paper the reliability of the method is investigated to establish likely error limits on the values of the couplings. The effects of strong coupling, isotopic labeling, phase variations, thermal noise, and t 1 noise are all considered. It is concluded that the procedure is robust with respect to deviations of the spectra from the ideal and, for typical experimental data, yields values of coupling constants which have errors of less than 10%.

Measuring proton shift tensors with ultrafast MAS NMR

A new proton anisotropic-isotropic shift correlation experiment is described which operates with ultrafast MAS, resulting in good resolution of isotropic proton shifts in the detection dimension. The new experiment makes use of a recoupling sequence designed using symmetry principles which reintroduces the proton chemical shift anisotropy in the indirect dimension. The experiment has been used to measure the proton shift tensor parameters for the OH hydrogen-bonded protons in tyrosine·HCl and citric acid at Larmor frequencies of up to 850 MHz.